Due to the limited availability and rising cost of petroleum, it is becoming increasingly desirable to develop nonpetroleum energy sources and to maintain a stable supply thereof. The more effective utilization of coal, which would greatly reduce the dependence of the world economy on petroleum, is now being enthusiastically reconsidered. However, since coal is a solid, its use presents transportation and handling difficulties not encountered with petroleum.
In order to facilitate handling and use of coal, coal-water slurries (.tbd.CWS"), consisting of fine particles of coal dispersed in water, have been developed. The amount of water necessary to form a pumpable slurry varies with the surface characteristics and extent of comminution of the coal. A coal slurry made up of particles, most of which will pass through a 200 mesh Tyler sieve can only contain from about 40 to 45 weight percent coal. When the coal content of a slurry (without additives) is increased much beyond this level, the viscosity of the slurry rises markedly and the slurry becomes unpumpable. To disperse the coal particles in water, achieve a high solids content slurry, and maintain an acceptable slurry viscosity, it is necessary to use a surfactant to reduce the coal/water surface tension and thereby facilitate the wetting of particle surfaces. See, for example, U.S. Pats. Nos. 4,302,212, Yamamura et al., issued Nov. 24, 1981; 4,304,572, Wiese et al., issued Dec. 8, 1981; and 4,104,035, Cole et al., issued Aug. 1, 1978; and 4,282,006, Funk, issued Aug. 4, 1981. All of the surfactants cited in the previous patents are of the anionic type, which the prior art generally recognizes as the preferred type of surfactant for use in dispersing solid particles.
Although the surfactants which have heretofore been used in coal slurries are effective in dispersing the coal particles and reducing the initial viscosity of the slurry, they are relatively ineffective in keeping the particles suspended in the slurry, i.e., the particles tend to settle out of the slurry due to gravitational forces if the slurry is not kept in motion. Often, this settling results in "hard packing" of the coal beneath a separated layer of water, a very undesirable occurrence. To avoid or delay settling, it has been generally found necessary to include a suspending agent (e.g., clays, starches or gums) in addition to the surfactant in the composition.
The object of the present invention is to provide superior surfactants for use in coal slurries, and, in particular, to provide surfactants which perform both dispersing and suspending functions in the slurries. A further objective is to provide surfactants which do not add nitrogen, sulfur or ash-producing elements (sodium or other metals) to the coal slurry.